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Maria Kashtalyan - One of the best experts on this subject based on the ideXlab platform.
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Evolution of crack density in cross-ply Laminates. Application of a coupled stress and energy criterion.
2016Co-Authors: Maria Kashtalyan, I.g. García, Vladislav ManticAbstract:Failure of continuous fibre-reinforced composite Laminates subjected to in-plane loading involves sequential accumulation of various type of damage. The first damage mode to appear is transverse cracking, i.e. matrix cracking in the off-axis plies of the Laminate. Initially, this type of damage is almost imperceptible in its influence on the global stiffness of the Laminate. However, transverse cracking promotes development of more dangerous types of damage such as delaminations which can grow from the tips of matrix cracks and significantly affect stiffness and strength of the Laminate. The combination of these factors makes very relevant the ability to accurately predict the onset and evolution of transverse cracking. Prior to the initiation of delamination and other damage mechanisms promoted by transverse cracks, a sequence of transverse crack onsets occurs. The transverse crack density has a great influence on the subsequent failure steps. Thus, the objective of this work is the prediction of the evolution of crack density when increasing the external load. The relation between the evolution of crack density and the external load is predicted by the combination of the Coupled Criterion of the Finite Fracture Mechanics (FFM) (Leguillon, Eur. J. Mech. A Solids 21:61–72, 2002) and the Equivalent Constraint Model (ECM) (Kashtalyan and Soutis, J. Mater. Sci.,41:6789-6799, 2006). The coupled criterion has already been used for the prediction of the critical strain leading to the first transverse crack onset in cross-ply Laminates (Garcia et al, Int. J. Solids Struct., 51:3844–3856, 2014). Thanks to the ECM, which provides analytical expressions for the energy released by the onset of a transverse crack in a Damaged Laminate, the applications of the coupled criterion can provide a quasi-analytical expression relating the evolution of crack density and the external load. The obtained results show a similar behavior to that found in experiments in the literature. Below a critical strain, the crack density is almost null. Above this critical strain a steep growth of the crack density is predicted in accordance with the experimental results. In addition, a strong effect of the transverse-ply thickness on the critical strain is predicted for thin and medium-thickness Laminates. However, this size effect is not found above a certain value for the transverse-ply thickness. The steep growth of crack density above this critical strain is also strongly affected by the transverse-ply thickness.
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predicting residual stiffness of cracked composite Laminates subjected to multi axial inplane loading
Journal of Composite Materials, 2013Co-Authors: Maria Kashtalyan, Constantinos SoutisAbstract:This is a contribution to the exercise that aims to benchmark and validate the current continuum damage and fracture mechanics methodologies used for predicting the mechanical behaviour of fibre-reinforced plastic composites under complex loadings. The paper describes an analytical approach to predict the effect of intra- (matrix cracking and splitting) and inter-laminar (delamination) damage on the residual stiffness properties of the Laminate, which can be used in the post-initial failure analysis, taking full account of damage mode interaction. The approach is based on a two-dimensional shear lag stress analysis and the equivalent constraint model of the Damaged Laminate with multiple Damaged plies. The application of the approach to predicting degraded stiffness properties of a multidirectional Laminate with multilayer intra- and inter-laminar damage is demonstrated for [0/90/0] and [0/908/0] cross-ply Laminates made from a specific glass/epoxy system under in-plane uniaxial and biaxial loading damage...
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Stiffness and fracture analysis of Laminated composites with off-axis ply matrix cracking
Composites Part A-applied Science and Manufacturing, 2007Co-Authors: Maria KashtalyanAbstract:Matrix cracks parallel to the fibres in the off-axis plies is the first intralaminar damage mode observed in Laminated composites subjected to static or fatigue in-plane tensile loading. They reduce Laminate stiffness and strength and trigger development of other damage modes, such as delaminations. This paper is concerned with theoretical modelling of unbalanced symmetric Laminates with off-axis ply cracks. Closed-form analytical expressions are derived for Mode I, Mode II and the total strain energy release rates associated with off-axis ply cracking in [0/θ] s Laminates. Stiffness reduction due to matrix cracking is also predicted analytically using the Equivalent Constraint Model (ECM) of the Damaged Laminate. Dependence of the degraded stiffness properties and strain energy release rates on the crack density and ply orientation angle is examined for glass/epoxy and carbon/epoxy Laminates. Suitability of a mixed mode fracture criterion to predict the cracking onset strain is also discussed.
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stiffness loss due to transverse cracking and splitting in cross ply Laminates under biaxial loading
Proceedings of ICCM12. 1999;:5-9., 1999Co-Authors: Maria Kashtalyan, Constantinos SoutisAbstract:In an effort to evaluate the stiffness degradation of composite cross-ply Laminates due to matrix cracking both in the 90 and 0 plies, a new theoretical approach was developed. It employs the Equivalent Constraint Model (ECM) [1, 2] and subsequent an improved 2-D shear lag analysis to determine stress field in the Damaged Laminate. In-situ Damage Effective Functions (IDEFs), used to describe stiffness reduction of the cracked or split lamina, were found to depend explicitly upon the crack density in the Damaged lamina and implicitly upon the crack density of the neighbouring lamina. Theoretical predictions for CFRP and GFRP Laminates revealed significant reduction in the shear modulus and Poisson's ratio due to splitting. Comparison of the new approach with existing models has shown reasonable agreement.
Janis Varna - One of the best experts on this subject based on the ideXlab platform.
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Crack Separation Based Models for Microcracking
Comprehensive Composite Materials II, 2018Co-Authors: Janis VarnaAbstract:Intralaminar cracking in layers of multidirectional Laminates lead to reduced ability of these layers to carry load which is the reason for Damaged Laminate thermo-elastic properties reduction. The average stress reduction in the Damaged layer is uniquely linked with displacements of the crack faces and, therefore, the crack opening displacements (CODs) and the crack face sliding displacements (CSDs) are alternative descriptors in Damaged Laminate stiffness predictions. In this chapter, exact closed form relationships are established linking thermo-elastic constants of the Damaged Laminate with crack density in layers and local parameters of the crack (average normalized COD and CSD). These robust local parameters depend on surrounding and Damaged layer stiffness and thickness ratio-relationship, which is investigated numerically and described by simple fitting functions. It is also shown that the energy release rate (ERR) for propagation of the intralaminar crack in steady-state conditions has simple expression through the COD and CSD, leading to simple predictive tool for thin ply Laminates where the cracking is propagation controlled.
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Effective stiffness concept in bending modeling of Laminates with damage in surface 90-layers
Composites Part A: Applied Science and Manufacturing, 2016Co-Authors: Andrejs Pupurs, Janis Varna, Mohamed Sahbi Loukil, H. Ben Kahla, David MattssonAbstract:Abstract Simple approach based on Classical Laminate Theory (CLT) and effective stiffness of Damaged layer is suggested for bending stiffness determination of Laminate with intralaminar cracks in surface 90-layers and delaminations initiated from intralaminar cracks. The effective stiffness of a layer with damage is back-calculated comparing the in-plane stiffness of a symmetric reference cross-ply Laminate with and without damage. The in-plane stiffness of the Damaged reference cross-ply Laminate was calculated in two ways: (1) using FEM model of representative volume element (RVE) and (2) using the analytical GLOB-LOC model. The obtained effective stiffness of a layer at varying crack density and delamination length was used to calculate the A, B and D matrices in the unsymmetrically Damaged Laminate. The applicability of the effective stiffness in CLT to solve bending problems was validated analyzing bending of the Damaged Laminate in 4-point bending test which was also simulated with 3-D FEM.
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validation of fem based Damaged Laminate model measuring crack opening displacement in cross ply Laminate using electronic speckle pattern interferometry espi
6th International Conference on Composites Testing and Model Identification Aalborg Denmark 22-24 April 2013, 2013Co-Authors: Mohamed Sahbi Loukil, Janis Varna, Zoubir AyadiAbstract:Composite Laminates during service undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode and the one examined ...
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Parameter determination in Damaged Laminate model by optical full-field measurement of the displacement using ESPI
2011Co-Authors: Mohamed Sahbi Loukil, Janis Varna, Zoubir AyadiAbstract:Parameter determination in Damaged Laminate model by optical full-field measurement of the displacement using ESPI
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On Effective Properties of Layers in Damaged Composite Laminates
Key Engineering Materials, 2011Co-Authors: Janis VarnaAbstract:Methodology is suggested for calculation of effective elastic constants of layer containing intralaminar cracks. Trends are presented for layers with low and medium high crack densities, where the interaction between cracks can be neglected. The effective properties of the Damaged layer are determined by back- calculation using previously developed GLOB-LOC model for thermo-mechanical properties of Damaged Laminate [1]. It is shown that the effective transverse modulus and shear modulus of a layer decrease linearly with increasing crack density, whereas longitudinal modulus and major Poisson’s ratio do not change at all. Experimental data for multiple cracking are analyzed showing that linear approximation of crack density versus applied strain may have sufficient accuracy. These two linear trends are used to calculate the effective elastic properties of a layer as a function of strain.
Daniel H. Cortes - One of the best experts on this subject based on the ideXlab platform.
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Stiffness reduction and fracture evolution of oblique matrix cracks in composite Laminates
Annals of Solid and Structural Mechanics, 2010Co-Authors: Daniel H. Cortes, Ever J. BarberoAbstract:A constitutive model to predict the onset and evolution of matrix cracking and the subsequent stiffness reduction is derived analytically. The formulation is valid for symmetric Laminates with otherwise arbitrary stacking sequence and matrix cracks in one or two directions. The proposed model calculates the reduction of the mechanical properties of the Damaged Laminate as function of crack densities. The onset and evolution of matrix cracks are predicted by the model in terms of unDamaged lamina properties and the critical strain energy release rates for modes I and II (G_IC and G_IIC). Therefore, there is not need for postulating damage evolution functions and no need for empirically adjusting the associated material parameters. The model formulation was specialized for the particular case of unidirectional loading. Comparison with experimental data showed an excellent prediction of crack initiation and evolution for a variety of Laminate stacking sequences. The combination of constitutive and damage evolution equations formed an integrated, mechanistic damage model with no adjustable parameters.
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A mechanistic model for transverse damage initiation, evolution, and stiffness reduction in Laminated composites
Composites Part B: Engineering, 2010Co-Authors: Ever J. Barbero, Daniel H. CortesAbstract:A constitutive model to predict stiffness reduction due to transverse matrix cracking is derived for laminae with arbitrary orientation, subject to in-plane stress, embedded in Laminates with symmetric but otherwise arbitrary Laminate stacking sequence. The moduli of the Damaged Laminate are a function of the crack densities in the damaging laminae, which are analyzed one by one. The evolution of crack density in each lamina is derived in terms of the calculated strain energy release rate and predicted as function of the applied load using a fracture mechanics approach. Unlike plasticity-inspired formulations, the proposed model does not postulate damage evolution functions and thus there is no need for additional experimental data to adjust material parameters. All that it is needed are the elastic moduli and critical energy release rates for the laminae. The reduction of lamina stiffness is an integral part of the model, allowing for stress redistribution among laminae. Comparisons with experimental data and some results from the literature are presented.
Wilfried Becker - One of the best experts on this subject based on the ideXlab platform.
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Repair of Laminates by External Patches — Analysis and Optimisation
PAMM, 2003Co-Authors: Heiko Engels, Wilfried BeckerAbstract:The present contribution considers the patch repair of a Damaged Laminate where the Damaged Laminate material has been removed by cutting or drilling a hole. The respective analytical assessment of such a repair configuration can be performed by the complex potential method. In order to determine the optimal repair design the present work also addresses the well-approved techniques of mathematical structural optimisation.
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Closed-form analysis of external patch repairs of Laminates
Composite Structures, 2002Co-Authors: Heiko Engels, Wilfried BeckerAbstract:The increasing use of composite material leads to the question of appropriate maintenance and repair techniques for composite structures. A preferred repair technique applies adhesively bonded patches, after the Damaged Laminate material has been removed by cutting or drilling a hole. In the present paper within the scope of classical Laminate theory, the problem of a Laminate plate with an elliptical hole repaired by elliptical patches under in-plane and/or bending load is investigated. For the considered base plate and repair patch arbitrary bending extension coupling is allowed. There are no restrictions for the elastic membrane and bending stiffnesses of the underlying layups. With regard to the hole and patch design any size of the elliptical shape is admitted. The analysis of the repair problem is performed by the complex potential method. The complex potentials are defined by appropriate series representations for the base plate, the repair region and the hole domain. The derived closed-form analytical solution provides all essential field quantities for an accurate and effective assessment of the external patch repair.
Ever J. Barbero - One of the best experts on this subject based on the ideXlab platform.
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Stiffness reduction and fracture evolution of oblique matrix cracks in composite Laminates
Annals of Solid and Structural Mechanics, 2010Co-Authors: Daniel H. Cortes, Ever J. BarberoAbstract:A constitutive model to predict the onset and evolution of matrix cracking and the subsequent stiffness reduction is derived analytically. The formulation is valid for symmetric Laminates with otherwise arbitrary stacking sequence and matrix cracks in one or two directions. The proposed model calculates the reduction of the mechanical properties of the Damaged Laminate as function of crack densities. The onset and evolution of matrix cracks are predicted by the model in terms of unDamaged lamina properties and the critical strain energy release rates for modes I and II (G_IC and G_IIC). Therefore, there is not need for postulating damage evolution functions and no need for empirically adjusting the associated material parameters. The model formulation was specialized for the particular case of unidirectional loading. Comparison with experimental data showed an excellent prediction of crack initiation and evolution for a variety of Laminate stacking sequences. The combination of constitutive and damage evolution equations formed an integrated, mechanistic damage model with no adjustable parameters.
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A mechanistic model for transverse damage initiation, evolution, and stiffness reduction in Laminated composites
Composites Part B: Engineering, 2010Co-Authors: Ever J. Barbero, Daniel H. CortesAbstract:A constitutive model to predict stiffness reduction due to transverse matrix cracking is derived for laminae with arbitrary orientation, subject to in-plane stress, embedded in Laminates with symmetric but otherwise arbitrary Laminate stacking sequence. The moduli of the Damaged Laminate are a function of the crack densities in the damaging laminae, which are analyzed one by one. The evolution of crack density in each lamina is derived in terms of the calculated strain energy release rate and predicted as function of the applied load using a fracture mechanics approach. Unlike plasticity-inspired formulations, the proposed model does not postulate damage evolution functions and thus there is no need for additional experimental data to adjust material parameters. All that it is needed are the elastic moduli and critical energy release rates for the laminae. The reduction of lamina stiffness is an integral part of the model, allowing for stress redistribution among laminae. Comparisons with experimental data and some results from the literature are presented.
